The aim of this project is to further our understanding of the way in which polyoma virus causes malignant transformation. Expression of the "early" region of polyoma virus is required to initiate and maintain the transformed state. The early region of polyoma virus codes for three related proteins (T antigens) -- large T antigen, medium T antigen and small T antigen. The large T antigen is required to initiate transformation but not usually for maintenance. Both the medium and the small T antigens appear to play roles in maintaining the transformed state. We plan to purify each of the three T antigens, both from polyoma virus transformed cells, and, if possible, from bacteria expressing recombinant DNA clones containing copies of the mRNAs for the three T antigens. We propose to investigate the structures and test the functions of the purified T antigens. In particular we will examine the role of the large T antigen in the initiation of viral DNA synthesis. In immunoprecipitates containing polyoma virus T antigens, there is preferential phosphorylation of the medium T antigen upon addition of labeled ATP. We will ask whether the purified medium T antigen itself possesses a protein kinase activity or whether there is a cellular protein kinase specifically associated with the medium T antigen. This is especially important to determine because it is already known that the transforming protein of Rous sarcoma virus is a protein kinase. The predicted amino acid sequence of the small T antigens has homology with certain glycoprotein hormones. We will measure whether the purified small T antigens has growth factor activity. In this manner we hope to define the ways in which the transforming proteins of polyoma virus interact with the cell to cause malignant transformation.